In the art of centrifugal throwing wheels, it is well-known how to lengthen the blast stream pattern. Conventional throwing wheels include a series of throwing vanes fixedly secured to a rotatable runnerhead, an impeller fixedly secured centrally of the runnerhead, and an impeller case which circumscribes the impeller and is provided with an opening through which the impeller causes blast media to be moved into locations such that the blast media is picked up by a respective throwing vane adjacent a heel portion thereof and accelerated as the blast media slides outwardly along the throwing vane toward the tip thereof. It is well-known that the tightest, or shortest pattern is achieved if the opening in the impeller case is a rectangular slot; changing the shape of the opening to a modified, or triangular slot spreads the pattern. The reason for this is that the triangular or similar slot causes the particulate, blast media to be deposited on the heel of the throwing vane over a longer period of time and this results in the blast media coming off of the tip of the vane also over a longer period of time and therefor produces a longer pattern.
There are times when it is desirable to shorten the pattern that results from using a conventional rectangular slot. If the particulate blast media is put on the heel of the throwing vane in a smaller or tighter package, it will also discharge over a shorter arc of rotation and result in a tighter or shorter pattern. Various methods have been tried in order to shorten the deposited package. For example, using more throwing vanes than the conventional eight vanes will result in a smaller package being placed on each vane. Also, reducing the circumferential dimension of the rectangular opening has been tried and it is thought to shorten the pattern in some instances; however, reduction of the rectangular opening necessarily limits the amount of blast media that can pass therethrough.
U.S. Pat. No. 3,290,827, granted on Dec. 13, 1966 discloses a throwing wheel which exemplifies the prior art to which the present invention particularly relates.